Third Generation, 3G, Networks such as Universal Telecommunication Network, UMTS, and CDMA 2000 provide high-speed wireless Internet access to mobile users over a wide coverage area. For the 3G networks the IP Multimedia Subsystem (IMS) has been defined to provide cellular access to telephony and multimedia services. The IMS uses packet-based technology, in particular IP, Internet Protocol, and other IETF, the Internet Engineering Task Force, protocols for offering services. The strength of IMS is offering of enhanced services, for example multimedia services combining voice and data. Further, the usage of IP-network as a single underlying standard allows an easy and fast service deployment.
Second Generation, 2G, networks, like Global System for Mobile communications, GSM, and 3rd generation networks, like UMTS, provide voice based services on a circuit-switched technology.
Currently the majority of terminals do not support IMS technology with SIP, Session Initiation Protocol, as defined in IETF-RFC 3261 as signalling protocol, as these terminals are based on circuit switched voice. Operators however want to give these 2G and 3G terminals access to their advanced IMS services.
In current art cooperation between 2G or 3G with an IMS is known as overlay system which allows 2G or 3G non SIP supporting terminals, calling, being called or during a call, mid-call, to make use of IMS based services.
The following sections describe simplified network architecture of IMS. In particular the nodes being involved in provision of service to non SIP supporting terminals in IMS architecture are mentioned as well as the involved parts of the 2G or 3G network.
The components of the IMS system are the CSCF, Call Session Control Function, the MGW/MGCF, Media Gateway/Media Gateway Control Function, the HSR, Home Subscriber Register, the AS, Application Server, the AP, IMS service application, and the SIM, Service Interaction Manager.
The applicable components in the mobile circuit switched network are the MSC, Mobile Service switching Centre, HLR, Home Location Register, VLR, Visitor Location Register, and RAN, Radio Access Network.
The CSCF acts as a call server and handles call signalling, it supports and controls the multimedia sessions and performs address translation functions. The CSCF can be functionally decomposed to S-CSCF, I-CSCF and P-CSCF.
The P-CSCF, Proxy-CSCF, is the first contact point in a visited IMS network and it provides authorization of bearer resources, further it forwards a SIP register request received from the UE, User Equipment, to an I-CSCF determined using the home domain name, as provided by the UE. In the opposite direction it forwards the SIP request or response to the UE. Moreover the CSCF forwards SIP messages received from the UE to an S-CSCF whose name the P-CSCF has received as a result of the registration procedure.
The I-CSCF, Interrogating-CSCF, is the contact point within an operator's network for all connections destined to a subscriber of that network operator. There may be multiple I-CSCFs within an operator's network. A function performed by the I-CSCF is assigning an S-CSCF to a user performing SIP registration.
The S-CSCF, Serving-CSCF, is the node that performs the session management for the IMS network. There can be several S-CSCFs in the network. The main functions of S-CSCF include: IMS session handling, acceptation of registration requests from UE, and interaction with an optional SIM for the support of services. Further it provides endpoints for specific services, like for example notification of tones/announcement, allocation of additional media resources, billing notification.
The HSR, Home Subscriber Register, is the centralized subscriber database. The HSR interfaces with the I-CSCF and the S-CSCF to provide information about the location of the subscriber and the subscriber's subscription information. The HSR is responsible for holding the following user related information: user identification, numbering and addressing information, user security information for authentication and authorization. The HSR supports the user registration, and stores inter-system location information.
The MGCF, Media Gateway Control Function, performs the inter-working between IMS and legacy circuit switched networks, like GSM. An IMS network may contain several MGCF nodes. The actual media, e.g. voice, is passed trough an MGW, Media Gateway. One MGCF controls one or more MGWs.
The MGCF performs protocol conversion between cellular call control protocols and IMS protocols. For example, the MGCF receives a SIP message from the CSCF and converts this message into appropriate ISUP, ISDN user Part, message and sends this ISUP message to the MSC. Thus, the primary function of MGCF is to convert signalling information from one format to another. This will predominantly be between ISUP, commonly used in circuit switched networks and SIP as used in IMS.
Like the MGCF for signalling, the MGW can provide comparable conversion for the media. This will predominantly be between PCM, Pulse Code Modulation, commonly used in circuit switched networks and an IP based format like used in the IMS. The MGW may also provide additional facilities for the IMS like secured tunnelling or firewall.
The actual IMS services are provided by APs, IMS service applications, running on the AS, Application Server. The AS is connected to the S-CSCFs in the IMS network through a SIM, Service Interaction Manager. The SIM takes care of chaining multiple services as well as solving inconsistencies when invoking multiple services. Additionally the SIM may take care that required information is extracted only once from registers like the HSR and passed to multiple services.
Terminals in the IMS network can establish voice calls using SIP as the signalling protocol. The SIP messages are sent to communicate the request to the CSCF, Call Session Control Function, in the IMS. In this case, the media is transmitted as packets throughout the network.
3G users have a subscription that is contained in the HSR. In order to access any service in IMS the user has to perform a registration procedure for his terminal in the IMS network. Said registration procedure is performed by means of a user agent implemented in the user's terminal.
Registrations in the IMS network have only a limited live time. They have to be constantly renewed, in practice every 24 to 48 hours. The user agent takes care of the renewal.
Terminals not being SIP compliant lack the user agent and need therefore an alternative way of subscription and registration in the IMS network. The alternative way is described with a GSM terminal as example.
Before the GSM terminal can be registered in the IMS network it requires that it is has a subscription in IMS. This subscription can either be fixed or dynamic. In the case of a fixed subscription the GSM terminal has a subscription comparable to SIP compliant terminals in the HSR which contains user and terminal details. In the case of a dynamic subscription a subscription is prepared in the HSR but the user/terminal details are dynamically entered in the registration procedure of the terminal.
A GSM subscriber has a subscription contained in a HLR. When a terminal of the GSM subscriber is switched on in a GSM network, the GSM terminal is registered in a VLR of that GSM network. The HLR holds the identity of the VLR/MSC where the subscriber is registered.
The HLR also holds for the GSM terminal which services are available in the IMS network by means of an ‘IMS service indicator’. The IMS service indicator is an indication that for a specific GSM terminal, specific services should be obtained from the IMS. This IMS service indicator may be an Intelligent Network, IN, service subscription element like an Originating CAMEL Subscription Information, O-CSI.
In this case signalling of a calling GSM terminal must be routed from the MSC to the MGCF, and subsequently to an S-CSCF in the IMS network to be able to invoke IMS services. Likewise, when being called, the signalling related to the terminating call must be routed from the GMSC to the MGCF, and subsequently to an S-CSCF.
As the GSM terminal is mobile it moves from one MSC to another which can be over network boundaries. The HLR has a specific mechanism of location update messaging to which functions can subscribe, who will then receive updates of change of location of the GSM terminal. Hereto the HLR instructs the VLR to send a notification to such functions when the user performs a location update. A comparable mechanism in VLR and HLR is present for switching the GSM terminal on or off.
The IMS network is in principle designed to deploy IMS services to registered terminals. The HSR holds all IMS registrations.
When a GSM terminal performs a dynamic registration in the IMS network, it is registered only for the period it is switched on, also known as “surrogate” registration.
In line with one of the basic IMS principles, Surrogate registrations in the IMS network have only a limited life time. They have to be constantly renewed, in practice every 24 to 48 hours, as long as the GSM terminal is switched on. A surrogate registration function in the IMS network takes care of the renewal of the surrogate registration.
The current art solution to register non SIP compliant terminals (not having the agent function) in the 3G as IMS users is based on a surrogate registration. This solution has however some drawbacks;
In practise every switched on non SIP compliant terminal remains registered in the IMS network for the event that it might invoke an IMS service. This gives an additional load on the IMS core network as well as the space required in the S-CSCF/HSR to store all surrogate registrations. The high number of registrations stored in the S-CSCF/HSR has also impact on the performance of the IMS network. The load on the IMS network is also high as it needs to perform the actions for registering non SIP compliant terminals, as well as act on expiring of the registration, and deregister when switched off.
Another problem in the current art is the loss of information like original called party number, International Mobile Subscriber Identity, IMSI, and location information. Due to incompatibility between ISUP and the SIP mechanism International Mobile Subscriber Identity and location information can not be included in the SIP Invite message for invoking a service in the IMS. In addition signalling constraints in ISUP may have the effect that the original called party number can be transported to the IMS network only partly.